One class of light-emissive displays is that using an organic material for light emission. The basic structure of these devices is a light emissive organic layer, for instance a film of a poly(p-phenylenevinylene) (“PPV”) or polyfluorene, sandwiched between a cathode for injecting negative charge carriers (electrons) and an anode for injecting positive charge carriers (holes) into the organic layer. The electrons and holes combine in the organic layer generating photons. In WO 90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline) aluminum (“Alq3”). In a practical device one of the electrodes is transparent, to allow the photons to escape the device.
A typical organic light-emissive display is fabricated on a glass or plastic substrate coated with a transparent anode such as indium-tin-oxide (“ITO”). A layer of a thin film of at least one electroluminescent organic material covers the first electrode. Finally, a cathode covers the layer of electroluminescent organic material. The cathode is typically a metal or alloy and may comprise a single layer, such as aluminum, or a plurality of layers such as calcium and aluminum.
In operation, holes are injected into the device through the anode and electrons are injected into the device through the cathode. The holes and electrons combine in the organic electroluminescent layer to form excitons which then undergo radiative decay to give light (in light detecting devices this process essentially runs in reverse).
Organic light-emissive displays can provide a particularly advantageous form of electro-optic display. They are bright, colorful, fast-switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. Organic (which here includes organometallic) light-emissive displays may be fabricated using either polymers or small molecules in a range of colors (or in multi-colored displays), depending upon the materials used.
Organic light-emissive displays may be deposited on a substrate in a matrix of pixels to form a single or multi-color pixellated display. A multicolored display may be constructed using groups of red, green, and blue emitting pixels. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel whilst passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image.
These devices have great potential for displays. However, there are several significant problems. One is to make the device efficient, particularly as measured by its external power efficiency and its external quantum efficiency. Another is to optimize (e.g. to reduce) the voltage at which peak efficiency is obtained. Another is to stabilize the voltage characteristics of the device over time. Another is to increase the lifetime of the device.
To this end, numerous modifications have been made to the basic device structure described above in order to solve one or more of these problems.
FIG. 1 shows a vertical cross section through an example of an OLED device 100. In an active matrix display, part of the area of a pixel is occupied by associated drive circuitry (not shown in FIG. 1). The structure of the device is somewhat simplified for the purposes of illustration.
The OLED 100 comprises a substrate 102, typically 0.7 mm or 1.1 mm glass but optionally clear plastic, on which an anode layer 106 has been deposited. The anode layer typically comprises around 150 nm thickness of ITO (indium tin oxide), over which is provided a metal contact layer, typically around 500 nm of aluminum, sometimes referred to as anode metal. Glass substrates coated with ITO and contact metal may be purchased from Corning, USA. The contact metal (and optionally the ITO) is patterned as desired so that it does not obscure the display, by a conventional process of photolithography followed by etching.
A substantially transparent hole conducting layer 108a is provided over the anode metal, followed by an electroluminescent layer 108b. Banks 112 may be formed on the substrate, for example from positive or negative photoresist material, to define walls 114 into which these active organic layers may be selectively deposited, for example by a droplet deposition or inkjet printing technique. The wells thus define light emitting areas or pixels of the display.
A cathode layer 110 is then applied by, say, physical vapour deposition. The cathode layer typically comprises a low work function metal such as calcium or barium covered with a thicker, capping layer of aluminum and optionally including an additional layer immediately adjacent the electroluminescent layer, such as a layer of lithium fluoride, for improved electron energy level matching. Mutual electrical isolation of cathode lines may be achieved through the use of cathode separators (element 302 of FIG. 3b). Typically a number of displays are fabricated on a single substrate and at the end of the fabrication process the substrate is scribed, and the displays separated. An encapsulant such as a glass sheet or a metal can is utilized to inhibit oxidation and moisture ingress.
Organic LEDs of this general type may be fabricated using a range of materials including polymers, dendrimers, and so-called small molecules, to emit over a range of wavelengths at varying drive voltages and efficiencies. Examples of polymer-based OLED materials are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of dendrimer-based materials are described in WO 99/21935 and WO 02/067343; and examples of small molecule OLED materials are described in U.S. Pat. No. 4,539,507. The aforementioned polymers, dendrimers and small molecules emit light by radiative decay of singlet excitons (fluorescence). However, up to 75% of excitons are triplet excitons which normally undergo non-radiative decay. Electroluminescence by radiative decay of triplet excitons (phosphorescence) is disclosed in, for example, “Very high-efficiency green organic light-emitting devices based on electrophosphorescence” M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, and S. R. Forrest Applied Physics Letters, Vol. 75(1) pp. 4-6, Jul. 5, 1999.” in the case of a polymer-based OLED, layers 108 comprise a hole conducting layer 108a and a light emitting polymer (LEP) electroluminescent layer 108b. The electroluminescent layer may comprise, for example, around 70 nm (dry) thickness of PPV (poly(p-phenylenevinylene)) and the hole conducting layer, which helps match the hole energy levels of the anode layer and of the electroluminescent layer, may comprise a conductive organic material, for example, around 50-200 nm, preferably around 150 nm (dry) thickness of PEDOT:PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene).
FIG. 2 shows a view from above (that is, not through the substrate) a portion of a three-color active matrix pixellated OLED display 200 after deposition of one of the active color layers. The figure shows an array of banks 112 and wells 114 defining pixels of the display.
FIG. 3a shows a view from above of a substrate 300 for inkjet printing a passive matrix OLED display. FIG. 3b shows a cross-section through the substrate of FIG. 3a along line Y-Y′.
Referring to FIGS. 3a and 3b, the substrate is provided with a plurality of cathode undercut separators 302 to separate adjacent cathode lines (which will be deposited in regions 304). A plurality of wells 308 is defined by banks 310, constructed around the perimeter of each well 308 and leaving an anode layer 306 exposed at the base of the well. The edges or faces of the banks are tapered onto the surface of the substrate as shown, heretofore at an angle of between 10 and 40 degrees. The banks present a hydrophobic surface in order that they are not wetted by the solution of deposited organic material and thus assist in containing the deposited material within a well. This is achieved by treatment of a bank material such as polyimide with an O2/CF4 plasma as disclosed in EP 0989778. Alternatively, the plasma treatment step may be avoided by use of a fluorinated material such as a fluorinated polyimide as disclosed in WO 03/083960.
As previously mentioned, the bank and separator structures may be formed from resist material, for example using a positive (or negative) resist for the banks and a negative (or positive) resist for the separators; both these resists may be based upon polyimide and spin coated onto the substrate, or a fluorinated or fluorinated-like photoresist may be employed. In the example shown the cathode separators are around 5 μm in height and approximately 20 μm wide. Banks are generally between 20 μm and 100 μm in width and in the example shown have a 4 μm taper at each edge (so that the banks are around 1 μm in height). The pixels of FIG. 3a are approximately 300 μm square but as described later, the size of a pixel can vary considerably, depending upon the intended application.
The deposition of material for organic light emitting diodes (OLEDs) using ink jet printing techniques are described in a number of documents including, for example: T. R. Hebner, C. C. Wu, D. Marcy, M. H. Lu and J. C. Sturm, “Ink-jet Printing of doped Polymers for Organic Light. Emitting Devices”, Applied Physics Letters, Vol. 72, No. 5, pp. 519-521, 1998; Y. Yang, “Review of Recent Progress on Polymer Electroluminescent Devices,” SPIE Photonics West: Optoelectronics '98, Conf. 3279, San Jose, January, 1998; EP O 880 303; and “Ink-Jet Printing of Polymer Light-Emitting Devices”, Paul C. Duineveld, Margreet M. de Kok, Michael Buechel, Aad H. Sempel, Kees A. H. Mutsaers, Peter van de Weijer, Ivo G. J. Camps, Ton J. M. van den Biggelaar, Jan-Eric J. M. Rubingh and Eiav I. Haskal, Organic Light-Emitting Materials and Devices V, Zakya H. Kafafi, Editor, Proceedings of SPIE Vol. 4464 (2002). Ink jet techniques can be used to deposit materials for both small molecule and polymer LEDs.
A volatile solvent is generally employed to deposit organic electronic material, with 0.5% to 4% dissolved material. This can take anything between a few seconds and a few minutes to dry and results in a relatively thin film in comparison with the initial “ink” volume. Often multiple drops are deposited, preferably before drying begins, to provide sufficient thickness of dry material. Typical solvents which have been used include cyclohexylbenzene and alkylated benzenes, in particular toluene or xylene; others are described in WO 00/59267, WO 01/16251 and WO 02/18513; a solvent comprising a blend of these may also be employed. Precision ink jet printers such as machines from Litrex Corporation of California, USA are used; suitable print heads are available from Xaar of Cambridge, UK and Spectra, Inc. of NH, USA. Some particularly advantageous print strategies are described in the applicant's UK patent application number 0227778.8 filed on 28 Nov. 2002 and the applicant's PCT patent application WO 2004/049466 filed on 18 Nov. 2003.
Inkjet printing has many advantages for the deposition of materials for molecular electronic devices but there are also some drawbacks associated with the technique. As previously mentioned the photoresist banks defining the wells may be tapered to form a shallow angle, typically around 15°, with substrate. However it has been found that dissolved organic electronic material deposited into a well with shallow edges dries to form a film with a relatively thin edge. FIGS. 4a and 4b illustrate this process.
FIG. 4a shows a simplified cross section 400 through a well 308 filled with dissolved material 402, and FIG. 4b shows the same well after the material has dried to form a solid film 404. In this example the bank angle is approximately 15° and the bank height is approximately 1.5 μm. As can be seen a well is generally filled until it is brimming over. The solution 402 has a contact angle θc with the plasma treated bank material of typically between 30° and 40° for example around 35°; this is the angle the surface of the dissolved material 402 makes with the (bank) material it contacts, for example angle 402a in FIG. 4a. As the solvent evaporates the solution becomes more concentrated and the surface of the solution moves down the tapering face of a bank towards the substrate; pinning of the drying edge can occur at a point between the initially landed wet edge and the foot of the bank (base of the well) on the substrate. The result, shown in FIG. 4b, is that the film of dry material 404 can be very thin, for example of the order of 10 nm or less, in a region 404a where it meets the face of a bank. When such a film is driven, only these thin outer portions emit light. This is due to the fact that the thinner areas have a lower resistance. As such, the current tends to flow only through these low resistance areas.
In practice drying is complicated by other effects such as the coffee ring-effect. With this effect because the thickness of solution is less at the edge of a drop than in the centre, as the edge dries the concentration of dissolved material there increases. Because the edge tends to be pinned solution then flows from the centre of the drop towards the edge to reduce the concentration gradient. This effect can result in dissolved material tending to be deposited in a ring rather than uniformly. In the situation where the central portion is thin and the outer ring is thick, then stronger emission will occur in the central portion. Driving a larger current through a smaller area results in a less efficient device.
The physics of the interactions of a drying solution with a surface are extremely complicated and a complete theory still awaits development.
Another drawback of banks with a long-shallow taper is that an inkjet droplet that does not fall exactly into a well but instead lands in part on the slope of the bank can dry in place, resulting in non-uniformities in the end display.
One solution to the aforementioned problems is to modify the bank structure as described in the present applicant's earlier application GB-A-0402559.9.
Another problem associated with ink jet printing of organic opto-electrical devices such as those discussed above is that in the resultant device, the organic hole injecting layer can extend beyond the overlying organic semi-conductive layer providing a shorting path between the cathode and the anode at an edge of the well.
One solution to the aforementioned problem is to modify the bank structure by, for example, providing a stepped bank structure which increases the length of the shorting path, thus increasing the resistance of the path resulting in less shorting. Such a solution has been proposed by Seiko Epson. However, providing a more complex bank structure is expensive and increases the complexity of the manufacturing method for the device.
The feasibility of using ink jet printing to define hole conduction and electroluminescent layers in OLED display has been well demonstrated. The particular motivation for ink jet printing has been driven by the prospect of developing scalable and adaptable manufacturing processes, enabling large substrate sizes to be processed, without the requirement for expensive product specific tooling.
The last five years have seen an increasing activity in the development of ink jet printing for depositing electronic materials. In particular there have been demonstrations of ink jet printing of both hole conduction (HC) and electroluminescent (EL) layers of OLED devices by more than a dozen display manufacturers. A number of these companies have set up pilot production facilities and have indicated that mass manufacture will start in 2007-2008 timeframe [M. Fleuster, M. Klein, P. v. Roosmalen, A. de Wit, H. Schwab. Mass Manufacturing of Full Colour Passive Matrix and Active Matrix PLED Displays. SID Proceedings 2004, 4.2].
The key reasons for the interest in ink jet printing are scalability and adaptability. The former allows arbitrarily large sized substrates to be patterned and the latter should mean that there are negligible tooling costs associated with changing from one product to another since the image of dots printed on a substrate is defined by software. At first sight this would be similar to printing a graphic image—commercial print equipment is available that allow printing of arbitrary images on billboard sized substrates [Inca digital website: http://www.incadigital.com/]. However the significant difference between graphics printers and display panels is the former use substrates that are porous or use inks that are UV curable resulting in very little effect of the drying environment on film formation. In comparison, the inks used in fabricating OLED displays are ink jet printed onto non-porous surfaces and the process of changing from a wet ink to dry film is dominated by the drying environment of the ink in the pixel. Since the printing process involves printing stripes (or swathes) of ink (corresponding to the ink jet head width) there is an inbuilt asymmetry in the drying environment. In addition OLED devices require the films to be uniform to nanometer tolerance. It follows that to achieve scalability and adaptability requires control of the film forming properties of the ink and a robustness of this process to changes in pixel dimensions and swathe timing. In this application it is demonstrated how this can be achieved with suitable ink engineering.
In general terms, the behavior of drying drops of HC and EL inks is explained by the coffee-ring effect first modeled by Deegan [R. D. Deegan, O. Bakajin, T. F. Dupont, G. Huber, S. R. Nagel, and T. A. Witten. Capillary flow as the cause of ring stains from dried liquid drops. Nature 389, 827 (1997)]. For the case of circular pixels the wet ink forms a section of a sphere, where the angle made by the drop surface with the substrate is the contact angle. When pinning occurs (which it invariably does for the inks and surfaces used in polymer OLED display manufacturing) the drying drop maintains its diameter and solute is carried to the edges of the drop forming a ring of material at the outer edges of the pixel. The amount of material carried to the edge depends on a number of factors—in particular how long the process of material transfer can occur before the drying drop gels and the uniformity of the drying environment. At a swathe edge more drying occurs on the unprinted side since the solvent concentration in the atmosphere above the substrate is less that the printed side. With more evaporation taking place on the unprinted side more solute is deposited on this side and the film profile becomes asymmetric.
The aforementioned problems are addressed in the applicant's earlier GB patent application no. 0510382.5 filed on 20 May 2005 by adapting compositions for ink jet printing comprising conductive or semi-conductive organic material. These adapted compositions are of particular use in the manufacture of light-emissive displays and produce films having an even thickness. A more uniform film morphology produces a more uniform emission in the finished device. However, it can be difficult and time consuming to design a suitable ink in order to produce sufficiently flat films for high quality light-emissive displays.
To summarize the previous discussion of the background to the invention, various problems are caused by non-uniform film formation in light-emissive displays. This is a particular problem when using ink jet printing of organic materials, although the problem also exists to a greater or lesser extent using other deposition techniques. Accordingly, a large amount of work has been carried out in order to try and achieve more even films by either modifying the structure of the ink wells and/or by modifying the composition of the inks. One problem with both these approaches is that the complex ink formulations, bank structures and deposition techniques required to achieve flat film formation can be difficult and expensive to manufacture. Furthermore, for each new material, such as a new electroluminescent polymer, the ink composition and/or bank structure is required to be modified in order to achieve optimal film formation and this is very time consuming and costly. Also, after an ink composition has been optimized for producing the best performance characteristics (e.g. electrical and optical properties), it may be difficult to modify the composition for obtaining optimal film formation without detrimentally affecting the performance characteristics. In light of these problems there is an on going need to provide new solutions to the problem of uneven film formation in light-emissive devices.
EP 0788014, EP 0949603, U.S. Pat. No. 6,020,869 and WO 99/42983 disclose light-emissive displays in which the pixels, or a sub-pixels, are split into a plurality of smaller discrete light emissive regions for improving grey-scale, each discrete light-emissive region having a single anode and a single cathode associated therewith. Multiple sub-pixels, e.g. three red sub-pixels, also provide degeneracy such that if one sub-pixel fails, the others can still function. However, the provision of multiple sub-pixels results in a lower aperture ratio causing a reduction in the lifetime of the display (the aperture ratio is the proportion of the actual emitting area to the total area of pixel, and the smaller the actual emitting area the harder it must be driven thus reducing lifetime). Furthermore, each discrete light-emissive region has the aforementioned problems of non-uniform film formation.
In light of these problems there is an ongoing need to provide new solutions to the problem of providing improved grey scale and degeneracy while retaining a high aperture ratio and lifetime for the display.
Another prior art display is shown in FIG. 5 and comprises parallel strips of red, green and blue emissive material (R, G, B) sandwiched between cathodes (C) running parallel to the strips of emissive material and anodes (A) running perpendicular to the emissive strips. Each strip of emissive material has one cathode and a plurality of anodes associated therewith. Each pixel has one anode and three cathodes associated therewith to form three discrete emissive regions (red, green and blue sub-pixels). Each sub-pixel has one cathode and one anode associated therewith. Each discrete light-emissive region suffers from the aforementioned problems of film non-uniformity and the sub-pixels do not have any degeneracy and may have poor grey-scale.
Embodiments of the invention seek to solve the aforementioned problems without having to design a suitable ink and a suitable bank structure in order to achieve flat film formation within a discrete emissive region of a pixel or sub-pixel.
Furthermore, embodiments of the invention seek to solve the aforementioned problems of providing degeneracy and/or improved grey scale without significantly reducing the aperture ratio of the display.